Driver circuit

ABSTRACT

A driver circuit ( 300 ) having a laser diode ( 306 ) coupled in series between a supply voltage ( 302 ) and a collector electrode of a transistor ( 310 ). The laser diode ( 306 ) has a low input impedance whereas the driving transistor ( 310 ) has a high output impedance. Resonances are caused by the impedance mismatch and reactive components. Therefore, an impedance feedback ( 320 ) is coupled between the laser diode input and the transistor data input to minimize the effects of the resonances.

FIELD OF THE INVENTION

[0001] The invention relates, in general, to driver circuits and moreparticularly to laser driver circuits, such as those used intelecommunications applications, especially, for high speed opticalcommunications.

BACKGROUND OF THE INVENTION

[0002] A laser driver circuit is used for modulating the light output ofa laser diode at high frequencies in accordance with data applied to thedriver circuit. The laser driver circuit is usually based on a balanceddifferential pair with a differential data input being applied to thedifferential pair.

[0003] However, various features associated with laser driver circuitslead to resonances which are problematic. For example, the impedancemismatch between the laser driver output and a laser, especially coupledwith reactive elements between them, can cause resonances. Resonancesmodify signals within a circuit, and may distort the output signal ofthe laser.

[0004] In order to attempt to overcome the resonance problems, severalmethods are employed by prior art devices. For example, it is known touse resistors in series to attempt to dampen the resonances, but passiveresistive components are lossy and often cannot be used due to limitedvoltage headroom in the laser driver circuit. Alternatively, it is knownto use reactive components or tuning stubs to match input and outputimpedances and reduce the resonances, but the reactive components ortuning stubs must be of a very high quality, which increases the cost ofmanufacture, and finely tuned, which limits the range of operatingfrequencies.

[0005] Therefore, it is desirable to remove, or at least reduce,resonances in a driver circuit without adversely affecting the output ofthe driver circuit.

BRIEF SUMMARY OF THE INVENTION

[0006] Accordingly, a first aspect of the present invention provides adriver circuit comprising a first transistor having a first currentelectrode coupled to a current source, a second current electrode and acontrol electrode coupled to a first input terminal, a low impedanceload having a first terminal coupled to the second current electrode ofthe first transistor and a second terminal coupled to a supply rail, andan impedance element coupled to the control electrode of the firsttransistor to provide AC feedback thereto.

[0007] Preferably, the first transistor forms part of a differentialpair in which a second transistor has a first current electrode coupledto the current source, a second current electrode coupled to a resistiveelement and a control electrode coupled to a second input terminal, thefirst and second input terminals being differential inputs for receivingdifferential data signals.

[0008] The impedance element preferably has a first terminal coupled tothe control electrode of the first transistor and a second terminalcoupled to the second current electrode of one of the first or secondtransistors. In a preferred embodiment, the driver circuit furthercomprises a second impedance element having a first terminal coupled tothe control electrode of the second transistor and a second terminalcoupled to the second current electrode of the second transistor.

[0009] In one embodiment, the impedance element comprises a differentialamplifier having an input coupled to the second current electrode of oneof the first or second transistors and a first and second differentialoutputs, the first differential output being coupled to the controlelectrode of the first transistor and the second differential outputbeing coupled to the control electrode of the second transistor.

[0010] Alternatively, the impedance element comprises a resistiveelement and a capacitive element coupled in series between the secondcurrent electrode of the first transistor and the control electrode ofthe first transistor.

[0011] The impedance element can include an inductive element.

[0012] Preferably, the low impedance load is a laser diode.

[0013] According to a second aspect of the present invention, there isprovided a method of mitigating for the effects of resonances in adriver circuit having a high impedance source and a low impedance load,the method comprising coupling an impedance feedback path between aninput to the low impedance load and an input to the high impedancesource.

[0014] Preferably, the low impedance load is a laser diode.

[0015] The high impedance source is preferably a first transistorforming part of a differential pair.

[0016] In a preferred embodiment, the impedance feedback path is aresistive element and a capacitive element coupled in series.

[0017] The values of the resistive element and the capacitive elementcan be chosen so as to provide a low pass filtering effect having acut-off frequency less than a lowest frequency of any substantialresonances present.

[0018] Alternatively, the values of the resistive element and thecapacitive element can be chosen so as to provide a filtering effectreducing output power including any resonances present but maintaining ahigh bandwidth.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Exemplary embodiments of the present invention will now bedescribed with reference to the accompanying drawings, of which:

[0020]FIG. 1 is a circuit diagram of a prior art laser driver circuit;

[0021]FIG. 2 shows an example frequency response of the prior art laserdriver circuit of FIG. 1;

[0022]FIG. 3 is a circuit diagram of a laser driver circuit constitutinga first embodiment of the present invention;

[0023]FIG. 4 shows an example frequency response of the laser drivercircuit of FIG. 3 with one set of component values;

[0024]FIG. 5 shows an example frequency response of the laser drivercircuit of FIG. 3 with a second set of component values; and

[0025]FIG. 6 is a circuit diagram of a laser driver circuit constitutinga second embodiment of the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

[0026] Thus, FIG. 1 shows a known laser driver circuit 100, essentiallyhaving a differential pair configuration. A resistor 104 is coupled to apositive supply voltage rail 102. Also coupled to the supply voltagerail 102 is a laser diode 106. The resistor 104 is connected in serieswith a collector electrode of a first transistor 108. An emitterelectrode of the first transistor 108 is coupled to a current source112. In a symmetrical manner, an emitter electrode of a secondtransistor 110 is coupled to the current source 112. A first signalinput lead 116 is coupled to a base electrode of the first transistor108 and a second signal input lead 118 is coupled to a base electrode ofthe second transistor 110.

[0027] In operation, a high frequency input data signal is applied tothe second signal input lead 118 and an inverse input data signal isapplied to the first signal input lead 116. The first transistor 108 andthe second transistor 110 are matched. Furthermore, the resistor 104functions as a means of power dissipation and approximate load matching.Generally, an impedance mismatch occurs because the sources of the laserdriver circuit 100 are first transistor 108 and second transistor 110 ofapproximate resistance 1 kΩ, whereas the loads of the circuit are laserdiode 106 having equivalent AC resistance of approximately 5Ω andresistor 104 having resistance of typically 30Ω.

[0028] An example frequency response curve for the laser driver circuit100 of FIG. 1 is illustrated in FIG. 2. At relatively lower frequenciesthe output response 200 maintains a constant value 202 with respect tothe input frequency. At high frequencies, resonances 204 are evidentwhich would have a substantial effect on an output signal.

[0029] A circuit diagram of a first embodiment of the present inventionis illustrated in FIG. 3. In this case, the structure of the circuitdiagram 300 is the same as that of the circuit diagram shown in FIG. 1,with identical features to those of FIG. 1 having the same referencenumerals as those of FIG. 1, but with the prefix “3” instead of theprefix “1”. Thus, for example, resistor 104 in FIG. 1 is resistor 304 inFIG. 3. In circuit diagram 300 a feedback impedance 320 is coupledbetween the collector electrode of the second transistor 310 and thesecond signal input lead 318. The feedback impedance 320 generallycomprises a resistor and a capacitor.

[0030] In operation, the addition of the feedback impedance 320 acts tofeed back the reflected signals from the laser diode 306 and transistor310 into the second signal input lead 318. The magnitude of the resistorand the capacitor within feedback impedance 320 affect the frequencyresponse of the circuit. Thus, the feedback impedance 320 can mitigatethe resonances.

[0031] Furthermore, in circuit diagram 300 an optional feedbackimpedance 322 (indicated by a dotted line in FIG. 3) is coupled betweenthe collector electrode of the first transistor 308 and the first signalinput lead 316. The optional feedback impedance 322, generallycomprising a resistor and a capacitor, assists in maintaining the laserdriver circuit 300 in a balanced state. An example frequency responsecurve of the laser driver circuit 300 of FIG. 3 is illustrated in FIG. 4and in FIG. 5. The frequency response curve 400 of FIG. 4 results whenthe resistor and the capacitor of the feedback impedance 320 haverelatively low values (for example 600Ω and 5 pF, respectively). In thiscase, the resistor and capacitor serve to provide a low pass filter witha cut-off frequency determined to be just lower than the resonancefrequency. At lower frequencies the output 400 maintains a constantvalue 402. The output 400 is however cut-off before the very highfrequency range (and associated resonances) are reached. Thus, althoughthe bandwidth is reduced, the output power is maintained at lowerfrequencies.

[0032] The frequency response curve 500 of FIG. 5 results when theresistor and the capacitor of the feedback impedance 320 have relativelyhigh values (for example 2 kΩ and 20 pF, respectively). Again, at lowerfrequencies the output 500 maintains a constant value 502. However, theoutput power is somewhat reduced at lower frequencies due to the effectof the feedback impedance. This filtering effect also causes theresonances to be substantially reduced, as shown at resonance peaks 504.

[0033] Ideally, the feedback impedance would have a very high valuecapacitor and a low value resistor to provide essentially DC blockingwith a flat frequency response. However, the realisation of a very highvalue capacitor within an integrated circuit causes manufacturingdifficulties.

[0034] A circuit diagram of a second embodiment of the present inventionis illustrated in FIG. 6. In this case, the structure of the circuitdiagram 600 is the same as that of the circuit diagram shown in FIG. 1,with identical features to those of FIG. 1 having the same referencenumerals as those of FIG. 1, but with the prefix “6” instead of theprefix “1”. Thus, for example, resistor 104 in FIG. 1 is resistor 604 inFIG. 6. In circuit diagram 600 a differential amplifier 620 is coupledbetween the collector electrode of the second transistor 610 and thebase electrode of both the first transistor 608 and the secondtransistor 610.

[0035] In operation, the addition of the differential amplifier 620 actsto feed back the output driving signal from transistor 610 into thefirst and second signal input leads 616, 618, and has a similar effectto the feedback impedance 320 in FIG. 3.

[0036] In operation, the alternative implementation has the same effectas the first implementation described.

[0037] Advantageously, the present invention provides a system whichminimises or removes resonances in a driver circuit Also, the inventionis simple to implement and relatively cheap to realise.

[0038] The present invention is generally applicable to differentialamplifiers and is particularly applicable to laser driver devices.

[0039] Whilst the invention has been described above in respect ofseveral particular embodiments and implementations of a laser drivercircuit, it will be appreciated that the present invention is applicableto any differential pair suffering, or potentially suffering, fromresonance effects. Furthermore, it will be appreciated that the abovedescription has been given by way of example only and that a personskilled in the art can make modifications and improvements withoutdeparting from the scope of the present invention. For example, itshould be apparent that the feedback impedance could include aninductance, which would provide a tuned filtering effect to remove justthe resonance frequencies. However, it is difficult to realise inductorson a chip.

1. A driver circuit (300) comprising a first transistor (310) having afirst current electrode coupled to a current source (312), a secondcurrent electrode and a control electrode coupled to a first inputterminal, a low impedance load (306) having a first terminal coupled tothe second current electrode of the first transistor (310) and a secondterminal coupled to a supply rail (302), and an impedance element (320)coupled to the control electrode of the first transistor (310) toprovide AC feedback thereto.
 2. A driver circuit (300) according toclaim 1, wherein the first transistor (310) forms part of a differentialpair in which a second transistor (308) has a first current electrodecoupled to the current source (312), a second current electrode coupledto a resistive element (304) and a control electrode coupled to a secondinput terminal, the first and second input terminals being differentialinputs for receiving differential data signals.
 3. A driver circuitaccording to claim 2, wherein the impedance element (320) has a firstterminal coupled to the control electrode of the first transistor and asecond terminal coupled to the second current electrode of one of thefirst or second transistors.
 4. A driver circuit according to eitherclaim 2 to claim 3, further comprising a second impedance element (322)having a first terminal coupled to the control electrode of the secondtransistor (308) and a second terminal coupled to the second currentelectrode of the second transistor (308).
 5. A driver circuit accordingto claim 3, wherein the impedance element comprises a differentialamplifier (620) having an input coupled to the second current electrodeof the first transistor and first and second differential outputs, thefirst differential output being coupled to the control electrode of thefirst transistor (610) and the second differential output being coupledto the control electrode of the second transistor (608).
 6. A drivercircuit according to any one of claims 1 to 4, wherein the impedanceelement (320) comprises a resistive element and a capacitive elementcoupled in series between the second current electrode of the firsttransistor (310) and the control electrode of the first transistor(310).
 7. A driver circuit according to any one of claims 1 to 4,wherein the impedance element (320) includes an inductive element.
 8. Adriver circuit according to any preceding claim, wherein the lowimpedance load (306) is a laser diode.
 9. A method of mitigating for theeffects of resonances in a driver circuit (300) having a high impedancesource and a low impedance load, the method comprising coupling animpedance feedback path between an input to the low impedance load (306)and an input to the high impedance source (310).
 10. A method ofmitigating for the effects of resonances in a driver circuit (300)according to claim 9, wherein the low impedance load (306) is a laserdiode.
 11. A method of mitigating for the effects of resonances in adriver circuit (300) according to claim 10, wherein the high impedancesource is a first transistor (310) forming part of a differential pair.12. A method of mitigating for the effects of resonances in a drivercircuit (300) according to claim 11, wherein the impedance feedback pathincludes a resistive element and a capacitive element coupled in series.13. A method of mitigating for the effects of resonances in a drivercircuit (300) according to claim 12, wherein the values of the resistiveelement and the capacitive element are chosen so as to provide a lowpass filtering effect having a cut-off frequency less than a lowestfrequency of any substantial resonances present.
 14. A method ofmitigating for the effects of resonances in a driver circuit (300)according to claim 12, wherein the values of the resistive element andthe capacitive element are chosen so as to provide a filtering effectreducing output power including any resonances present but maintaining ahigh bandwidth.